In casting single crystal and columnar grain turbine blades using directional solidification techniques, ceramic cores are positioned in an investment shell mold to form internal cooling passageways in the cast turbine blade. During service in the gas turbine engine, cooling air is directed through the passageways to maintain blade temperature within an acceptable range.
As described by Frank et al. in U.S. Pat. No. 4,837,187 (Howmet Corporation), ceramic cores heretofore used in the casting of nickel and cobalt base superalloy turbine blades have comprised silica, zirconia, alumina, and yttria selected to be relatively non-reactive with the superalloy being cast so as not to react with reactive alloying components thereof, dimensionally stable during directional solidification (DS) when the superalloy melt is cast at high temperatures into a preheated shell mold and solidified about the core for extended times required for DS of single crystal or columnar grained microstructures, and also to be removable within reasonable times from the cast turbine blade by chemical leaching techniques.
In recent turbine blade designs, the cooling passageways are provided with complex serpentine configurations that in turn require a complex core shape. After the cast component is solidified, the mold and core are removed from the component. Typically, the ceramic core is chemically leached out of the cast component using a hot aqueous caustic solution so as to leave cooling passageways in the component.
After the mold and core are removed from the cast component, the component typically is subjected to a post-cast inspection procedure to determine if any residual ceramic core material remains in the cooling passageways after the core leaching operation. The inspection procedure may include neutron radiographic and/or x-ray radiographic techniques. In the neutron radiographic technique, the component is bathed in a Gd-containing solution to tag any residual ceramic core material that may reside in the cooling passageways. Since Gd is a strong neutron absorber, it will indicate the presence of any residual ceramic core material in the passageways during neutron radiography. If residual ceramic core material is detected, then the component is subjected to additional chemical leaching to remove the material.
An x-ray inspection procedure also can be used following removal of the mold and core as described in U.S. Pat. No. 5,242,007 wherein the ceramic core is either doped or tagged with an x-ray detectable agent and subjected to x-ray radiography to detect any residual ceramic core material in the passageways.
An object of the present invention is to provide a ceramic core that exhibits the aforementioned relative non-reactivity with the melt being cast, dimensional stability during solidification, chemical leachablity from the cast component, and enhanced x-ray detectability during post-cast inspection operations.